Abstract

An image intensified video camera was used in conjunction with the EISCAT radar in Tromsø, Norway, for detailed studies of non-sheared black aurora on March 5, 2002. While it is known that a downward field-aligned current will decrease the ionospheric plasma density, any measurable effect will depend on the noise-level in the density measurements and the background density. The results show that the plasma density is not reduced in the regions of black aurora, which shows that if a downward current existed within the black aurora, it must be less than ∼0.6 μA/m2. Combining this result with recent findings that small scale currents are generally strong, that the downward currents are associated with intense diverging electric fields, and that the observed black arcs show no signs of shear motion, it is concluded that the non-sheared black aurora is not associated with any significant downward field-aligned current.